Purification of silver nitrate solutions



Patented Oct. 14, 1952 I l I UNITED stares cram orrics PURIFICATION OFSILVER NITRATE SOLUTIONS Jerome A. Moede, New Brunswick, N. 1., assignorto 13.71. du, Pont de Nemours and Company, Wilmington, DeL, acorporation of Delaware No Drawing. Application February 21, 1951,

. Serial No. 212,211

g 5 Claims. (01. lace-1,02)

This invention relates to the purification of oxide to attain a pl-l' ofat least 6.1 and preferably;

aqueous silver nitrate solutions. Moreparticubetween 6.1 and 9.0 (the pHattained will, of larly it relates to the removal of contaminatingcourse, vary with the concentration of the silver metal ions fromaqueous solutions oi silver nie nitrate solution), (1)) allowing theprecipitated trate. 5 metals and metal hydroxides to separate out,Photographic silver halide emulsions are made (0) removing the silvernitrate solution and (d) by precipating silver halides in an aqueouscolpassing it into contact with one or more water.- loid medium. Theaqueous silver nitrate solu} insoluble, porous, solid adsorbents atleast one tions used as the source of the silver ,ions should of whichis a water-insoluble, porous, basi ads be very pure and free from othermetal salts in sorbent, e. g., activated alumina or magnesia. order toobtainoptimum photographic proper- The other adsorbent may be elementalcarbon ties. The prior art methods for the preparation including carbonblack and activated wood charof such solutions are q ite timecon'sumingand coal and preferably is used in fine granular form tedious andrequire a plurality. of crystallizahaving particle sizes ranging from10* to l tlons. l micron. In U. s. application Serial No. 125,166, filedW n wo difie nt adsor n s. a e us d t is November 2, 1949 which has beenissued as Patbest to first pass the aqueous silver nitrate soluent No..5%,792, March 6, 1951, there is detion which is recovered fromthesilver oxide scribed a process of purifying aqueous silver treatmentinto contact with the basic adsorbent nitrate solutions which comprises(1) passing the but the invention is not limited to this order ofaqueous silver nitrate solution into contact with procedure. Thepreferred order has the advancelemental carbon, (2) passing theresulting solutage that the elemental carbon adsorbent aids tion intocontact with activated alumina, and in removin y p r l s of l m n r mag-(3) filtering the resulting solution through a silnesia for instancewhich may be en rain in he ver oxide impregnated filter. The foreignmetal solution. It the event the elemental carbon adions aresubstantially completely removed and a sorbent treatment is eliminatedor the solution silver nitrate solution of photographic qualityrecovered is still cloudy or hazy because of the recovered. This processwhile, commercially presence of alumina or magnesia particles thepractical has the disadvantage that'the alum na solution'may be passedthrough a microporous must be replaced or.regeneratedcirequentlybefilter, e. g., a'stainless steel mcroporous material cause it isexhausted rapidly by the acid nature or an inert microporous clayceramic material or of the silver nitratesolution, and; the relativelyfritted glass. large quantities of metal impurities which must Step amay be advantageously carried out.

be removed. v i by adding. s lv r'o z to n aq eo olut n 3 An object ofthis invention is to provide an to 6 molars in silver nitrate whichcontains other improved process tor preparing silver nitrat heavy metalsalts as impurities until the pI-l of solutions having a high degree ofrity, I the solution has been raised above 6.1. The other object is toprovide a simplified process amount of silver oxide used will depend onthe for preparing silver nitrate, solutions of good pH of. the untreatedsolution which generally is photographic quality. A further object. isto w between 0.5 and 3.6. The precipitate formed and provide a processwhich is free from the disany excess silver oxide are then allowed tosettle advantages of the process described in the aforeout and theresulting silver nitrate solution is said application. A further objectis to provide drawn off and subjected to further treatment. such avprocess whichis economical and can be It. has been determined that thisstep removescarrled out on a relatively lar e 6- Still b-B byprecipitation as the hydroxides. or free metals ts W be app nt fr m.- tf ll win depractically all of the metal impurities which C D f the b ello behave as poisons in photographic emulsions and The OVe oblects areac mp sh nd t which are normally present in nitric acid anddisadvantages of prior art silver nitrate purisilver bullion. fi a p p oures o er me by he pro es o so The silver nitrate solution recoveredfrom step hi mvent n hlCh mp s (a) t n an (a) may then be brought intointimate contact aqueous solution. of silver n tr e h ch Q lt Il withparticles or pieces of elemental carbon prefther me a s and me al i nsas impurit es. eg. erably by passing it through a column packed coppe irn. lead. gold. bismuth, m rcu y. th with same. The amount of carbon maybe from plat n m metals? etc-I, Withvsufficient silver 0.02-0.l% byWeight of the silver nitrate in the solution to be purified. Thistreatment removes size of 10- micron, the alumina was 28-48 mesh ions ofmetals of the "platinum group, gold, and the stain-less steel filter hadan average pore bismuth and mercury and photographically size of 5microns. The temperature of the soluharmful organic impurities remainingafter the tion was 25-35 C. and the suction regulated so of fiow wasapproximately 250 and other solid impurities. 00., min. Afterpurification the pH was adjusted The aqueous siliver nitrate solutionwhich is to 3.0 with 1N nitric acid and the resulting solurecovered fromthe carbon treatment may then tion which was of high purity was used toprebe brought into intimate contact with particles pare high qualityphotographic emulsions. or pieces of activated alumina or magnesia pref-Data from the spectrographic analysis of the with same. In general,there should be from 2 process are summarized below:

Metal impurity grams per 10 grams silver nitrate Pd Au Cu Pb Bi Pt Hg IrRu Rh I Na less less less less less less less less less less less thanthan than than than than than than than than than 0.04 0.1 1.0 0.50.0025 0.25 0.15 0.02 0.04 0.04 60 also preferably in fine granular formhaving an The control emulsion was prepared from a C. P.

The aqueous silver nitrate solution recovered 3 developed at 68 F. for 3minutes in a developer from the activated alumina may sometimesconconsisting of 0.46 gram N-methyl-p-aminophenol tain fine particles ofalumina in suspension and sulfate, 3.4 grams hydroquinone, 28 gramssodithey can be removed by any practical method um sulfite. 10 grams ofthe mono-hydrate of soof removing fine particles from solution so thatdium carbonate, and 0.44 gram of potassium they will not cause spots inthe finished films or bromide all diluted up to one liter:

Test Emulsion Control Emulsion average pore size of 5 microns or less.

Rel. H 61 D Rel. H a D After any alumina or magnesia particles haveSpeed Gamma pee am a been removed, the pH of the silver nitrate solutioncan be adjusted to 3.0-3 5 with dilute nltrlc 52 88 -0 53 2.89 .01

EXAMPLE II precipitation of llght-sensltlve silver halides in 40 aphotographic colloid emulsion Example I was followed exactly except forthe The invention will be further illustrated but that the treatment wasentirely is not intended to be limited by the following ehminatedfromthe Proms- The resulting ilver examples nitrate solution was of highquality and used EXAMPLE I directly to make high quality photographicemulsions.

Forty-three liters of an aqueous solution con- Data from thespectrographic analysis of the taining '65 kilograms of silver nitratep'repared silver nitrate solution obtained by the above from C. P.nitric acid and 99.97% silver bullion process are summarized below:

Metal impurity grams 'per 10 grams silver nitrate Pd An Cu Pb Bi Pt HgIr Ru Rh N;

M x N x less less less less less less less less less less less than thanthan than than than than than than than than 0.05 0.1 1.0 0.5 0.00250.25 0.15 0.02 0.04 0.04

l Summarized below are the results of sensitowith distilled water. Threehundred and fifty metric tests on positive typ gelatino iodo-brofor 15minutes and the solution was then passed manufactured by theconventional method of active carbon, packed on a 6 m. diameter filter,developed at 68 F. for 3 minutes in a developer 1400 grams of activatedalumina, packed in a 60 consisting of 0.46 gram N-methyl-p-aminophenolmm. diameter column, and a porous stainless sulfate, 3.4 gramshydroquinone, 28 grams sodisteel filter. The carbon had an averageparticle 7 um sulfite, 10 grams of the mono-hydrate of so-' dil mcarbonate. and O-A igram or potassium bro mi e e di ut up t 0118 li e?ControlEmulsion 6 that. P ss n lN: so ium hydroxide t ro gh t andfinally washing it with distilled water. Alumina can be coated withsilver oxide by passing sodium hydroxide solution through it until thepH oi," t'e filtrate is greater than 11 and then passing silver nitratesolution through it to precipitate s'lver oxide on the grains. Thematerial is then washed with distilled water to remove sodium ions andother foreign ions. The silver oxide coated alumina is used exclusivelyto prevent the silver nitratesolution from dissolving the'silver oxideimpregnated in the filter by affording exposure to silver oxide beforeit reaches the'filten;

' After purification the pI-I was adjusted to 3.0

with in c. P. min acid and the resulting solution which was Qf h'ghpurity'was used directly to prepare high q ality photographic emulsions.Datafrom thespectrographic analysis of the 20 silvernitrate solutionobtained by the above process are sunnnarized below:

Metal impurity grams. per-105 grams silver nitrate lessless than thanthan 0.5

Qu ,Ru 1 Rh Na less less less less less less less less less than thanthan than than than tllian than Summarized below are the results ofsensitometric tests on X-ray' type gelatino iodobromide emulsions having1.55 mol percent iodide. The control emulsion was prepared from a C. P.grade of silver nitrate commonly used in the manufacture of photographicemulsion and Metal impurity grams per grams silver nitrate Pd Au Cu PbB1 Pt Hg Ir Bu Rh Na less less less less less less less less less lessless than than than than than than than than than than than 0.05 0.1 1.00. 5 0.0025 0.25 0.15 0.02 0.04 0.04 60 EXAMPLEIV manufactured by theconventional method of A process was carried out exactly the same asExample III except that the filtration through the porous stainlesssteel filter was eliminated. The resulting silver nitrate solutionpossessed a degree of purity identical to that given in Example I11.

EXAMPLE V To 10.5 liters of 9M silver nitrate prepared from C. P. nitricacid and 99.9% silver bullion and having pH 0.5 was added a slurry ofsilver oxide prepared by direct precipitation from approximately 500 cc.of 3M silver nitrate and an equivalent amount of 3M sodium hydroxide andwashed five times by decantation. Distilled water was added to bring thevolume of the mixture up to 16 liters and the mixture stirred until thepH rose above 6.1.

The mixture was allowed to settle for minutes and the solution thenpassed by suction, in one step, through grams of active carbon, packedin a 6 in. diameter filter, 1400 grams of activated alumina packed in a60 mm. diameter column, and a filter containing 100 grams of the aluminacoated with silver oxide on a 90 mm. diameter fritted glass discimpregnated with silver oxide. The carbon and alumina were of the samegrade used in Example I. The fritted disc had a maximum pore size of 14microns. The fritted disc can be impregnated with silver oxide by firstsaturating it with 1N silver nitrate,

hydrate.of sodium carbonate and 4.5 grams of potassium bromide alldiluted up to one liter:

f g/ M h.

l E Test Emulsion Control Emulsion l Rel. H a D Bel. H a 1) Speed GammaFog Speed Gamma Fog as 2.14 .01 79 2. 57 .01

This invention has the advantage that it provides a practical processfor purifying aqueous silver nitrate solutions on a commercial scale. Afurther advantage resides in the fact that the first step alone ofadding sufiicient silver oxide to attain a pH of at least 6.1 removesmore than of the metal impurities present in silver nitrate obtainedfrom silver bullion and commercial nitric acid. The use of silver oxidebefore treatment with alumina and carbon black has the advantage thatthe alumina does not have to be replenished or regenerated as frequentlyand a smaller amount of this material can be used. Similarly, a smalleramount of carbon is required by the processes. In some cases 15 wherethe amount of organic impurities is very small the carbon adsorbent maybe eliminated. This reduces raw material costs, handling costs andenables one to reduce the size of the equipment. Yet another advantageresides in the fact that more uniform purified silver nitrate solutionscan be made from different lots of solutions treated. H

As many widely different embodiments of this invention can be madewithout departing from the spirit and scope thereof, it is to be understood that the invention is not-to be limited except as defined by theclaims. v What is claimed is: I 1. A process of purifying an aqueoussolution of silver nitrate containing small amounts of heavy metal andnoble metal impurities which comprises treating said solution withsilver oxide to attain a pH ofat least 6.1, separating the silvernitrate solution from the precipitate and passing the resulting solutioninto contact with at least one water-insoluble porous solid adsorbent,at least one of which is an adsorbent H taken from the group consistingof alumina and magnesia.

2. A process of purifying an aqueous solution of silver nitratecontaining small amounts 'of' heavy metal and noble metal impuritieswhich comprises treating said solution with silver oxide to attain a pHof at least 6.1, separating the solution from the precipitated metalsand metal hydroxides and silver oxide and passing the resulting solutioninto contact with at least one water-insoluble, porous solid adsorbentat least one of which is alumina. v i 1 3. A process of of silvernitrate heavy metal and containing small amounts of purifying an aqueoussolution noble metal impurities which tact with 0.02 to comprisestreating said solution with silver ox- W purifying an aqueous solutioncontaining small amounts of tact with activated alumina and bringing theresulting solution into intimate contact with carbon particles.

5. A process of of silver nitrate purifying an aqueous solutioncontaining small amounts of heavy metal and noble metal impurities whichcomprises treating said solution with silver oxide to attain a pH of atleast 6.1, allowing the precipitated metals, metal hydroxides and anypassing said solution into con- 0.1% by weight based on the silvernitrate of finely divided active carbon having particle sizes rangingfrom 10- to 1.0 micron and bringing the resulting solution into contactwith from 2 to 10% by weight based on the silver nitrate of finelydivided activated alumina having an average particle size of 28-100mesh.

JEROME A. MOEDE.

nitrate solution,

No references cited.

1. A PROCESS OF PURIFYING AN AQUEOUS SOLUTION OF SILVER NITRATECONTAINING SMALL AMOUNTS OF HEAVY METAL AND NOBLE METAL IMPURITIES WHICHCOMPRISES TREATING SAID SOLUTION, WITH SILVER OXIDE TO ATTAIN A PH OF ATLEAST 6.1, SEPARATING THE SILVER NITRATE SOLUTION FROM THE PRECIPITATEAND PASSING THE RESULTING SOLUTION INTO CONTACT WITH AT LEAST ONEWATER-INSOLUBLE POROUS SOLID ADSORBENT, AT LEAST ONE OF WHICH IS ANADSORBENT TAKEN FROM THE GROUP CONSISTING OF ALUMINA AND MAGNESIA.